The present invention relates to a construction of a tank or tanker for the transportation or storage of a liquid such as crude oil, chemicals or the like, and more particularly to a ship hold construction wherein a partition membrane or a membrane receptacle is employed to prevent the mutual contamination of two kinds of oils loaded in the tanker or the mutual contamination of loaded oil and ballast water, especially for preventing the ballast water from being contaminated by the oil. The invention is applicable to not only a tanker but also to other installations such as a petroleum storage tank or the like in the sea or a storage or transportation tank of other liquids.
It is well known to separate ballast water and loaded oil in a compartment of a tanker by an elastic partition membrane or a membrane receptacle made of rubber coated cloth or the like in order to prevent the ballast water from being contaminated by the oil. Such partition membranes are disclosed in Japanese Patent Application No. 48-29184 and Japanese Utility Model Application No. 51-23392, and such a membrane receptacle is disclosed in Japanese Patent Application No. 48-23187.
The conventional method of separating the ballast water and the oil disclosed in such applications will be briefly described. In FIGS. 1 and 2, a tank body or a ship held 1 is partitioned into an oil chamber 3 and a ballast water chamber 4 by securing a partition membrane 2 in a liquid tight manner along lines A-B, B-C, C-D and D-A of the tank 1. The chambers 3 and 4 are equipped with intake/exhaust ports 5 and 6 and gas exhaust pipes 7 and 8, respectively. Japanese Patent Application No. 51-20385 discloses a method wherein one surface of the membrane 2 is in contact with the ballast water while the other surface thereof is in contact with the oil during the charging and discharging of the water and the oil. FIGS. 2(a) to 2(e) show a process for the discharge of oil by a supply of ballast water, and FIGS. 2(f) to 2(i) show a process for the discharge of ballast water by supplying oil. In the case of a tanker, the process of FIGS. 2(a) to 2(e) is accomplished at the crude oil unloading base and the process of FIGS. 2(f) to 2(i) is done at the crude oil loading base.
A tank having such a partition membrane or membrane receptacles has a number of practical disadvantages, however, Firstly, in a stage of FIG. 2(d), for example, that is just before the oil is completely discharged, the partition membrane 2 is urged toward the upper surface of the tank 1 so that the intake/exhaust port 5 is clogged with or blocked by the membrane as shown in FIG. 3. As a result it is impossible to discharge the residual liquid from the tank. Further, if too much discharge force is applied the elastic material of the partition membrane, such as rubber coated cloth, is sucked into the intake/exhaust port 5 and the membrane may ultimately be broken. This is also true in the case of gas exhausting from the pipe 7. In order to eliminate this defect the intake/exhaust port could be moved to a more suitable position. Empirically, however, it is impossible to completely discharge the residual liquid from the tank 1 even when the position of the intake/exhaust port is appropriately selected. Alternatively, a plurality of intake/exhaust ports may be used. In this case, however, the space outside of the tank becomes unduly restricted whereby this arrangement of ports is impractical. Secondly, as shown in FIG. 4, unduly great tension is applied to portions 10 of the membrane 2 around the corners 9 or along the ridges of the tank 1 due to imperfect contact between the membrane 2 and the corners 9.